The setting features biomancers who, to summarize, are magic users who can create living things as minions or servants through various ways(seeds, modified eggs, whatever), have the option to directly control their creations like a puppeteer if they have line of sight, and can manipulate life in general, including but not limited to altering their own bodies/cells and the bodies/cells of other living things(if they can come into contact with them).
The effects of their magic can happen either slowly or quickly, the time to completion being chosen by the biomancer and a typical effect being the aquiring of claws, but the faster it happens the more pain is involved in the process. There is a high amount of biological knowledge required to do a great degree of anything in biomancy so it's not like a novice or unlearned will be able to do everything they want, but even the greenest of novice tends to know a reasonable amount of human anatomy and knows you can't live if your heart stops beating so try and stay away from their touch if you fight one.
Biomancers, though they are magic users, are a little more limited to the laws of physics than other magic users, being that their creatures and any biological modifications are subject to the square cube law and nutritional needs and all that, and unless they want to dedicate the time and effort to experimentation they tend to be limited to similar designs to what is already out there in the biosphere somewhere(after having discovered and studied/took them apart to see what makes them tick). The biosphere they have available to them is similar to ours, with the addition of magical creatures whose atypical workings are magical in nature and so won't be able to be able replicated by biomancers as whatever creature a biomancer makes is non-magical in nature.
Magic users in many settings have the ability to significantly and effectively contribute to a war effort or the siege of a fortified location, and so I want this to apply to biomancers as well. Problem is I don't know if they have the ability to do so as they are now, seeing as living things tend to be rather fragile and weak compared to mechanical war machines of basically any era or the good ol' large and heavy ram.
The 'ram' part is what will be the focus of this particular question or, at least, the ability to break down or through the gates of a gatehouse during a siege. Ideally the creature the biomancer designs in an attempt to have it be able to do this should still be alive afterwards in order to deal with other gates beyond the one it first deals with, but a one-shot suicidal creature will do as well, if a living thing is even capable of breaking down those gates, portcullis included.
With the ability to freely design creatures in mind, seeing as that might increase the odds of them being capable, my question basically boils down to... Are living things capable of breaking down a medieval gatehouse's gates?